Samsky:
Hi,My arduino project "basketball shooter" which aims to throw a rubber ball to a
rim by mechanical arm( at MAX distance = 30~50 cm ), I plan to use a three axis
arm and this means three servo motors are needed, what kind of servo motor should I
pick for making a rubber ball "fly" ? Does the weight of servo motor care?
I'd say you have a low chance of making this work with a hobby servo with any accuracy/repeatability.
You need something with smooth and controllable torque for well-defined launch, and hobby-servos
aren't upto the smooth and controllable torque bit. Certainly they may work well for setting up the
starting pose, but the throwing mechanism needs to be mechanically smooth, with as little variation
as possible.
(mechnical friction in a cheap multi-step gearbox is "poorly characterized" as they say - its not well
behaved. The position control loop inside the servo is subject to jitter and noise you cannot
measure or control.)
A belt reduction drive and fairly torquey (ie large) DC motor immediately sounds like a better
bet - you can control PWM and duration of action and leave the motor unbraked (make sure
your H-bridge supports this, most do). That ought to give a controllable and repeatable response.
The belt will reduce vibration and jerkiness of the motion (ie damp it). Remember a throw requires
a certain minimum power to work.
Of course if you want position control in the same axis as the throwing mechanism you'll need
to throw in an encoder and close the position control loop yourself (disabling it briefly for a throw).
The spring suggestion in #2 has a lot going for it too, but mechnically is complex (the release
mechanism is tricky). It allows the motors to be less powerful than the throw power too, keeping
motor size under control.
And finally a general note about robot arms - if you counterbalance each segment of the arm it is
much easier to control and much lower torques are needed at each joint. And the thing doesn't
buckaroo about when moving fast...
If you need extra reduction gearing a single stage of metal gears with light oil lubrication sounds
good to me (grease has a strong relation between viscosity and temperature).